Independent mobility, the ability of an individual to successfully and safely navigate through their environment, is a primary contributor to quality of life. Unfortunately, mobility often decreases following a stroke, preventing many stroke survivors from returning to typical levels of activity participation. A major contributor to reduced mobility is gait instability, which can limit function either by increasing the risk of falls or by increasing the fear of falling. Several existing rehabilitation techniques have been demonstrated to improve certain aspects of post-stroke gait function, such as increasing self-selected walking speed. However, these interventions have generally failed to address gait instability, as evidenced by a lack of improvement in fall risk. A likely reason for the limited effectiveness of current rehabilitation approaches is that they are not based on the unique mechanisms underlying post-stroke gait instability. A longer-term goal of this general line of research is to develop clinically-available, mechanism-based interventions to improve post-stroke gait stability. As a step toward accomplishing this goal, the project will focus on a potential mechanistic cause of instability suggested by preliminary work investigating how gait is stabilized among uninjured controls and stroke survivors. This approach contrasts with typical investigations of gait instability, which largely focus on quantifying non-causal indicators of stability rather than understanding the underlying mechanisms. The central hypothesis is that post-stroke disruptions in gait stability are often caused by a lack of mechanically-appropriate adjustments in foot placement location. While individuals with stable gait patterns actively control their foot placement based on the mechanical state of their center of mass, this evidence of active stabilization is often lacking after a stroke. The disruption of the typical gait stabilization strategy has motivated the recent construction of a prototype rehabilitation robotics device able to manipulate foot placement location. The objective of this proposal is to further develop this device and conduct initial testing of its ability to improve post-stroke gait stability, based on principles of motor learning. This will be accomplished through three Specific Aims. The first Specific Aim is to identify the simplest force-field control method able to effectively modulate foot placement. Several candidate control methods will be compared in terms of their ability to increase mechanically-appropriate foot placement modulation. The second Specific Aim is to determine whether the typical gait stabilization strategy can be restored by repeated gait practice while the force-field either: 1) encourages mechanically-appropriate foot placement; or 2) amplifies errors away from mechanically- appropriate target locations. These two intervention strategies are based on distinct theoretical frameworks. The ?challenge point framework? suggests that mechanical assistance provided by the force-field may allow stroke survivors to experience (and relearn) a movement pattern they may otherwise be unable to accomplish. In contrast, the ?error-driven adaptation? framework suggests that amplified kinematic errors may drive participants to actively resist these forces, producing beneficial after-effects in foot placement. The third Specific Aim will quantify the effects of patient baseline characteristics and dosage on the intervention?s effects, necessary data for future larger-scale trials. The proposed project is based on the combined theoretical frameworks of human gait mechanics and motor learning, and will quantify the potential of mechanism-based interventions using a novel force-field to restore the typical neuromechanical gait stabilization strategy in stroke survivors. The resultant knowledge has the potential to make an important contribution to the development of a larger-scale rehabilitation paradigm in which therapeutic interventions are targeted to a patient?s specific limitations.